Generally, in the field of X-ray computed tomography (CT) apparatuses, an X-ray CT apparatus that includes an X-ray generator to generate cone-beam X-rays and an X-ray detector capable of detecting the X-rays is known. The X-ray CT apparatus of this type can perform volume scanning that causes the X-ray generator to emit X-rays while rotating once along the circular path around a subject, so that projection data for one rotation is acquired from the X-ray detector. The X-ray CT apparatus of this type can also reconstruct, using the one-rotation projection data, a three-dimensional image having an area spreading in the direction of the rotational axis.
However, the range that allows for acquisition of one-rotation projection data is restricted and a field of view (FOV) on the respective axial plane becomes small as the distance from the mid-plane (the central plane defined with respect to the rotational-axis direction) increases. That is, the size of a normal area, where a complete set of one-rotation projection data is available, becomes small. Concurrently, as the normal area becomes small, the size of a mask area—present on both sides of the normal area and where a complete set of one-rotation projection data is not available—becomes large. Note that the mask area is an area which does not enable reconstruction processing with one-rotation projection data, thus resulting in the corresponding area in the resultant image becoming masked.
In this regard, a technique is put into practical use. This technique sees an image of the mask area reconstructed using the projection data acquired from the range smaller than that for one rotation. The technique will also be referred to as “mask area reconstruction”. According to this mask area reconstruction, a three-dimensional image, or its tomograms, can be generated by combining reconstructed images of the mask area and the normal area.
As such, the normal area and the mask area differ from each other in the range for acquiring projection data for reconstruction. This range for acquiring projection data corresponds to the range of the rotational angle of a gantry carrying the X-ray generator and the X-ray detector, the range of views, and so on. The difference in the range of the gantry's rotational angle means a difference in the range of time for acquiring projection data, as well as a difference in the averaged acquisition time (average time). Accordingly, a three-dimensional image or a tomogram that combines images of the normal area and the mask area may be understood to be a combination of images differing in the range for acquiring projection data (e.g., average acquisition time).
When the volume scanning is performed on a subject's site, which makes only a small or slow movement, the resultant image does not show a large gap at the boundary between an image of the normal area and an image of the mask area, even though the normal area and the mask area differ from each other in the average acquisition time.
On the other hand, when the volume scanning is performed on a site which is moving largely or quickly (an “actively moving site”), a large gap appears at the boundary between an image of the normal area and an image of the mask area due to the difference between the normal area and the mask area in the average acquisition time. Using a three-dimensional image or its tomograms including such a gap for assessing conditions of adhesion or infiltration to the periphery, etc., involves a risk of degrading assessment accuracy.
The objects intended by the embodiments include reducing the gap appearing at the boundary between an image of the normal area and an image of the mask area when an actively moving site is subject to volume scanning.